Upper critical solution temperature, miscibility

It should be noted that the modern view is that all partially miscible liquids should have both a lower and upper critical solution temperature so that all such systems really belong to one class. A closed solubility curve is not obtain in all cases because the physical conditions under normal pressure prevent this. Thus with liquids possessing a lower C.S.T., the critical temperature (the critical point for the liquid vapour system for each component, the maximum temperature at which liquefaction is possible) may be reached before the consolute temperature. Similarly for liquids with an upper C.S.T., one or both of the liquids may freeze before the lower C.S.T. is attained. 

We consider a binary liquid mixture of components 1 and 3 to be consistent with our previous notation, we reserve the subscript 2 for the gaseous component. Components 1 and 3 are completely miscible at room temperature the (upper) critical solution temperature Tc is far below room temperature, as indicated by the lower curve in Fig. 27. Suppose now that we dissolve a small amount of component 2 in the binary mixture what happens to the critical solution temperature This question was considered by Prigogine (P14), who assumed that for any binary pair which can be formed from the three components 1, 2 and 3, the excess Gibbs energy (symmetric convention) is given by... 

Note A miscibility gap is observed at temperatures below an upper critical solution temperature (UCST) or above the lower critical solution temperature (LCST). Its location depends on pressure. In the miscibility gap, there are at least two phases coexisting. 

The lower critical solution temperature is another crucial polymer property, which, together with the Upper Critical Solution Temperature (UCST), defines fhe fwo solubility boundaries of polymers in solution. Typically, systems are completely miscible below the LCST but only partially miscible above the LCST and completely immiscible above the UCST. 

The instance we have considered here, that of a polymer in a poor solvent, results in an upper critical solution temperature (UCST) as shown in Figure 2.33. This occurs due to (a) decreased attractive forces between like molecules at higher temperatures and (b) increased solubility. For some systems, however, a decrease in solubility can occur, and the corresponding critical temperature is located at the minimum of the miscibility curve, resulting in a lower critical solution temperature (LCST). This situation is illustrated in Figure 2.34. 

Chen et al. [67,68] further extended the study of binary blends of ESI over the full range of copolymer styrene contents for both amorphous and semicrystalline blend components. The transition from miscible to immiscible blend behavior and the determination of upper critical solution temperature (UCST) for blends could be uniquely evaluated by atomic force microscopy (AFM) techniques via the small but significant modulus differences between the respective ESI used as blend components. The effects of molecular weight and molecular weight distribution on blend miscibility were also described. 

Although the biphasic properties of fluorous-organic systems are desirable for separations, monophasic conditions would favour enhanced reaction rates. Therefore, it is important to know the general miscibilities of fluorous solvents and the effect of temperature (Tables 7.2 and 7.3). In Table 7.2, the temperature given for the phase separation is a consulate or upper critical solution temperature. However, these temperatures should only be taken as a guide, as... 

If 13 < A < 15, the solvents may be only partially miscible with an upper critical solution temperature (UCST) between 25 and 50°C. This is a borderline case. If the binary mixture is miscible, then adding a relatively small amount of water likely will induce phase splitting. 

Thin polymer films composed of two layers with different composition have been used for almost two decades to determine the diffusion coefficient [12,78-82] on the basis of observed broadening of their initial profiles ( >(z). When the two layers are built of two fully miscible phases (T>TC regime for blends with upper critical solution temperature UCST), a free interdiffusion takes place with the interface growing with time t as w1/2°=t1/2. This process proceeds without limits and results in a single homogeneous phase. 

This figure clearly shows the temperature and composition windows where it is either a two-phase system or a single-phase system. The characteristic features of an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST) corresponding to the phase transition are identified. For a particular composition of two immiscible polymers, if the temperature is increased, the UCST is the highest temperature at which two phases may co-exist in the blend. There is then a window of miscibility as the temperature is increased further, followed by phase separation again at the LCST. This type of diagram is often seen for polymer solutions, e.g. polystyrene in cyclohexane. Often polymer blends show... 

The separate phases will be rich in one component but may have the other present as a minor component. In order to control compatibility the elastomer may have reactive end groups to enhance interfacial adhesion. A common example in epoxy-resin technology is the carboxy-terminated butadiene-acrylonitrile copolymer (CTBN). The structure is shown in Scheme 1.47. In this resin the solubility in the epoxy resin is conferred by the acrylonitrile group, and an increase in the fraction present decreases the upper critical solution temperature, with 26% acrylonitrile conferring total miscibility of CTBN with a DGEBA-based epoxy resin (Pascault et al, 2002). 

The process of phase separation during cure arises from the change in the phase diagram as the cure reaction of the epoxy resin progresses. This is shown schematically in Figure 1.34 (Pascault et al, 2002) for a system with an upper critical solution temperature (UCST) in which the lower curve represents the system miscible at room temperature, with the fraction cpRo of elastomer corresponding to the initial composition of the rubber-epoxyresin system before any cure reaction has taken place. 

Cloud-point curves or precipitation curves for different polymer-solvent systems have different shapes (Figs. 3.12 and 3.13). The maxima and minima on these curves indicate the upper critical solution temperature (UCST) and the lower critical solution temperature (LCST), respectively. As indicated in Figs. 3.12 and 3.13, the phase diagram of a polymer solution has two regions of limited miscibility (i) below UCST associated with the theta temperature (see Problem 3.16) and (ii) above LCST. 

In general, the miscibility of a pair of polymers depends on temperature and composition. Figure 10.1 schematically shows three typical phase diagrams. The ordinate and the abscissa axes represent temperature and composition, respectively. The solid line in Fig. 10.1(a), below which the blend becomes immiscible (two-phase), is referred to as an upper critical solution temperature (UCST). However, Fig. 10.1(b) shows a lower critical solution temperature (LCST) behavior. Some polymer pairs display both UCST and LCST as shown in Fig. 10.1(c). As will be shown in the following, UCST is rarely observed for a polymer blend. 

From the above table it is clear that the lower critical solution temperature is raised, and the upper critical solution temperature is lowered, by increase of pressure. Under j>res,sure of 830 kgm. per sq. cm. the two critical solution points coincide. Under pressures higher than this, complete miscibility exists at all temperatures. A similar behaviour is found in the case of water and methylethylketone. 

Generally, liquid-liquid phase equilibrium (or phase separation) occurs only over certain temperature ranges, bounded above by the upper consolute or upper critical solution temperature, and bounded below by the lower consolute or lower critical solution temperature. These critical solution temperatures are indicated on the liquid-liquid phase diagrams given here. All partially miscible mixtures should exhibit either one or both consolute temperatures however, the lower consolute temperature may be obscured by the freezing of the mixture, and the upper consolute temperature will not be observed if it is above the bubble point temperature of the mixture, as vaporization will have instead occurred. ... 

If the binodal and spinodal points are determined at various temperatures and are plotted together, a phase diagram such as the one shown in Figure 6.1 b may result. The temperature at which the binodal and spinodal curves merge together is the critical temperature. The phase diagram shown illustrates a case in which the miscibility gap occurs at temperatures above the critical temperature, and the system is said to exhibit a lower critical solution temperature (LCST) behavior. A system, on the other hand, may display an upper critical solution temperature (UCST) behavior, in which the miscibility gap occurs below the critical temperature. 

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